Hot-dip galvanized (HDG) steel sheets have advantages such as manufacturing compared to electrocoating and low costs of products and therefore their uses are recently extending to broad areas such as household electric appliances and motor vehicles. However, in spite of their low costs, the hot-dip galvanized steel sheets have surface qualities inferior to those of electro-galvanized (EG) steel sheets and therefore are not widely used for applications in which distinctness of image (DOI) or favorableness of external appearance after painting is a very important factor, such as outer plates of motor vehicles or household electric appliances. Further, hot-dip galvanized steel sheets suffer from problems and disadvantages such as inferior corrosion resistance, blackening resistance and oil stain resistance, as compared to electro-galvanized steel sheets.
As such, in compliance with their extended uses, the hot-dip galvanized steel sheets are required to have superior quality characteristics in conjunction with favorable surface appearance comparable to that of electro-galvanized steel sheets, and in particular, there are required improvements in surface appearance, oil stain resistance and blackening resistance, which are inferior to those of electro-galvanized steel sheets.
Disadvantageous properties of the hot-dip galvanized steel sheets such as inferior surface appearance, corrosion resistance, oil stain resistance and blackening resistance, as compared to those of electro-galvanized steel sheets, result from coating layer-formation reactions and manufacturing processes of the hot-dip galvanized steel sheets. In electro-galvanization, the coating layer is composed of fine crystalline grain. Whereas, the coating layer obtained by hot-dip galvanization is composed of large crystalline grains. As a result, there is a difference in grain boundary therebetween. That is, the coating layer obtained by electro-galvanization is made up of fine crystallines having a size of several μm to several tens of μm, whereas the coating layer of the hot-dip galvanized steel sheet is susceptible to occurrence of a unique coating texture aspect, called a spangle or flower pattern, and the coating texture of commercially available hot-dip galvanized steel sheets generally has a texture region size of more than 500 μm.
Occurrence of such coarse spangles is due to characteristics of solidification reaction of zinc. That is, when zinc is solidified, dendrites in the form of the branches of a tree rapidly grow from a solidification nucleus as a starting point at an early stage of solidification, forming a skeletal structure of the coating texture, and thereafter a non-solidified molten zinc pool, which remained between dendrites, solidifies, thus resulting in completion of solidification reaction. That is, it can be said that the size of spangles is dependent on the size of skeleton of the coating texture which was determined at the early stage of solidification.
Further, when dendrites grow, since they solidify while consuming molten zinc present therearound, a region of dendrites convexly protrudes and a region of the pool concavely depresses, thereby resulting in a non-uniform thickness of the coating layer, i.e., occurrence of hills and valleys on the coating surface.
Further, upon solidification of molten zinc, features and forms of spangles vary depending upon what manner hexagonal crystal structures of zinc are crystallographically arranged on the surface of the steel sheet. In other words, one hot-dip galvanized layer is composed of various forms of zinc crystals (spangles), thus representing that hexagonal crystal structures of zinc are placed at different angles according to respective regions of the coating layer. Generally, crystal orientation in which a basal plane of zinc is placed parallel to the surface of the steel sheet is known to exert the most superior corrosion resistance, blackening resistance and chemical stability, but it is very difficult to make all of the spangles to have desired basal planes.
Consequently, each and every spangle in one hot-dip galvanized steel sheet has different crystal planes of zinc exposed to the surface and there are differences in chemical reactivity according to respective regions due to non-uniformity of crystal orientation, which are believed to result in inferior corrosion resistance, oil stain resistance and blackening resistance of the hot-dip galvanized steel sheet as compared to electro-galvanized (EG) steel sheets having uniform surface texture.
Meanwhile, generally in corrosion, the exterior of grains, grain boundary, has a high electrochemical potential and thereby serves as an anode where corrosion proceeds, whereas the interior of grains serves as a cathode. Where the area of the anode is relatively small as compared to that of the cathode, corrosion locally and rapidly progresses.
In a hot-dip galvanization process, when skin-pass rolling for improving surface appearance via securing of mechanical properties and inhibition of spangle exposure is carried out, adverse effects such as non-uniformity of crystal structures and occurrence of coarse coating texture are more pronounced. That is, each spangle exhibits a different degree of deformation caused by rolling, and as a result, adverse effects due to non-uniformity of crystal structures become even worse. Further, as the coarse coating texture exhibits more conspicuous shapes of dendrites, there are significant unevenness of surface profile according to respective regions of the coating layer. As a result, regions protruded upon skin-pass rolling are mechanically further deformed, resulting in serious problems associated with heterogeneous qualities according to respective regions.
In order to solve the above-mentioned shortcomings due to spangles and in order to obtain qualities comparable to those of electro-galvanized steel sheets, it is necessary to micronize spangles to the maximum extent possible. For such reasons, a variety of methods for decreasing the spangle size have been proposed.
For example, mention may be made of the following methods: (1) Method using a coating bath to which antimony (Sb) or lead (Pb) is not added, (2) Method involving performing skin-pass rolling after coating is complete, and (3) Method involving spraying water or an aqueous solution immediately before solidification of the zinc-coating layer.
However, the coating methods (1) and (3) may reduce the size of spangles, but suffer from difficulty to achieve a decrease of the spangle size equal to the level of electrocoating, due to a high solidification rate of zinc. Hereinafter, the reasons for that will be specifically described.
The first reason is based on solidification properties of molten zinc. That is, the steel sheet has a thickness of about 0.4 to 2.3 mm, whereas the hot-dip galvanized layer typically has a thickness of about 7 to 10 μm and does not exceed a maximum of 50 m, which is very thin as compared to the steel sheet.
As such, when the coating layer is solidified while being cooled, solidification of the coating layer takes some period of time because the steel sheet has a large amount of latent heat stored therein. At this time, dendrites grow in the surface direction of the steel sheet. Therefore, spangles having a size of about 0.5 to 1 mm occur even with combined use of Method 1 and Method 3, and it has been regarded by consumers of the steel sheet that such a size is almost free of spangles and is sufficient to be used in desired applications.
For consumers requiring favorable surface appearance, it is necessary to completely remove traces of spangling. For this purpose, the steel sheet is prepared by increasing an amount of skin-pass rolling in Method (2). Here, the coating layer is crushed by skin-pass rolling, resulting in elimination of surface heterogeneity such as spangling, and thereby it is possible to achieve surface qualities similar to the level of the electroplated material to some extent. However, since the coating layer is deformed by mechanical force, more skin-pass rolling leads to poor blackening resistance, oil stain resistance and corrosion resistance, thus presenting a problem of short-term storage of the steel sheet.
As a method of reducing the spangle size by controlling the solidification reaction of the coating layer, there is a method of solidifying the coating layer by vigorously spraying an aqueous solution at relatively high-pressure or by spraying a finely divided zinc powder upon solidification of the coating layer. However, high-pressure spray is likely to result in damaged appearance due to marks pitted by impingement of sprayed liquid droplets of the aqueous solution on the zinc-coating layer in a molten state. In addition, spraying of zinc powder suffers from problems such as environmental contamination due to scattering of zinc dust inside plants and dent defects on the steel sheet caused by sticking of zinc powder, which was not completely fixed thereon, to various rolls.
As techniques relating to spangle-free hot-dip galvanized steel sheets and manufacturing methods thereof, reference may be made to Japanese Patent Laid-Open Publication Nos. 1999-100653, 1985-181260 and 1982-108254, Korean Patent Laid-Open Publication No. 2001-57547 and EP 1348773 A1, which disclose a galvanized steel sheet having a spangle size of 10 to 88 μm. However, there is no disclosure on hot-dip galvanized steel sheets having no traces of dendrite solidification, control of aluminum content in the coating layer and control of height differences between hills and valleys in the coating layer. In addition, Korean Patent Laid-Open Publication No. 61451 and U.S. Pat. No. 4,500,561 disclose a method for minimization of spangling on hot dip galvanized steel strip by forming an electric field and passing liquid droplets through the electric field, but do not mention about fabricating a charged electrode into a mesh shape.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a hot-dip galvanized steel sheet having superior corrosion resistance, oil stain resistance and blackening resistance and exhibiting favorable surface appearance.
It is another object of the present invention to provide a spangle-free hot-dip galvanized steel sheet that can be used as a material for use in inner and outer plates of car body, household electric appliances and building materials and steel sheet for painting.
It is a further object of the present invention to provide a method of manufacturing a hot-dip galvanized steel sheet having superior corrosion resistance, oil stain resistance and blackening resistance and exhibiting favorable surface appearance.
It is yet another object of the present invention to provide a hot-dip galvanization hot-dip galvanized device for use in manufacturing a hot-dip galvanization steel sheet having superior corrosion resistance, oil stain resistance and blackening resistance and exhibiting favorable surface appearance.